I'm not sure an Apple Store will do anything, but an Apple Authorized Service Provider may depending on whether the laptop has a key cap replacement set available from Apple. However, not all laptops have this kit and the UK kit would likely only be available in the UK since it is a region specific build. If you ever need to have the Top Case Assembly replaced, then this could cause problems for you.
It may also be possible to purchase key caps online as well, but you will need to be very careful they are compatible with your keyboard. Apple many times will source parts from multiple vendors, so there may be more than one type of key cap available. Many times the difference is very subtle, but very significant none the less.
Plus it is easy to damage the key scissor or butterfly mechanism when removing or attaching a key cap, even an experienced tech can accidentally damage the mechanism since they are very easily damaged. While it is usually possible to replace a damaged scissor mechanism, a damaged butterfly mechanism requires replacing the entire Top Case Assembly which is very expensive as it includes the palmrest, keyboard, trackpad, and battery (also touchbar & speakers on some models).
Another option is to use vinyl stickers to cover the key caps with the characters you want, but this will likely affect the keyboard backlight (may or may not be an issue for you).
The lesson here is that you should buy the laptop with the setup you want since changing things after purchase is not usually easy or possible.